Radiation from a laser has been used, and still is being used, for cutting various materials including dielectric materials in general. Laser cutting is now increasingly used as a replacement for mechanical cutting of large sheets of glass into smaller shapes for various applications.
Several techniques have been developed over the past several years using various types of laser alone or in combination, i.e., with two or more lasers required for a particular process. The laser types used include long-wavelength infrared IR lasers, particularly carbon-dioxide (CO2) gas-discharge lasers, near IR (NIR) solid-state pulsed lasers, and pulsed, solid-state lasers with wavelengths in the visible and near ultraviolet (UV) region of the spectrum. This latter type typically involves NIR pulsed lasers with frequency conversion using optically nonlinear crystals.
The choice of a laser (or lasers) for the glass-cutting depends, inter alia, on the glass type and thickness of glass being cut, the cutting speed required, and the quality desired of a cut edge. Cost of the laser cutting equipment can also be a factor in deciding which type of laser and process to select.
In recent years there has been developed a type of glass generally categorized as chemically strengthened glass. This type of glass is in increasing demand for covering display screens on hand-held electronic devices such as “smart phones” and tablet computers.
The chemical strengthening is achieved by an ion-exchange process in which a sheet of conventional glass is immersed in a solution containing potassium (K) salts in a manner which causes potassium ions to replace sodium ions in outer surface regions of the glass. This places the outer surface regions of the sheet in compression, with an inside (center) region in tension. By way of example in a sheet of glass having a total thickness of about 0.5 millimeters (mm), the compressive surface regions have a depth of about 21 micrometers (μm). The surface-compression regions make the chemically strengthened glass particularly resistant to cracking or breakage due to mechanical stresses or impact. The compressive and tensile stresses are in a stable balance which provides the stress and impact resistance properties.
The compression-tension stresses also make this glass difficult to cut as most processes developed for conventional glass can upset the stable balance between the compressive and tensile stresses to some degree creating unwanted cracking or breakage and generally resulting in an unacceptably low cutting-yield. A most common approach to avoiding the problem of cutting the chemically strengthened glass is to cut desired shapes from glass prior to strengthening and then immerse the cut parts in the solution for strengthening individual parts. This however requires means for handling a high volume of individual parts during the strengthening process.
There is a need for a laser cutting method for chemically strengthened sheet-glass wherein the laser-radiation interaction with the chemically strengthened glass can be controlled in a manner which preserves the stable balance between the outer compressive stress and the inner tensile stress. The method should be applicable to cutting relatively large sheets of the chemically strengthened glass.